1. Field of the Invention
The present invention relates to a vehicle environmental control systems and methods. More particularly, the present invention relates to systems and methods for automatically sensing fogging conditions when the relative humidity is less than 10%.
2. Description of Related Art
In conventional automobile control systems, application of remedial measures to combat fogging of the vehicle windows is manually instituted by the vehicle passengers. Naturally, such remedial measures, which primarily consist of activating a vehicle heating, ventilation, and air conditioning (HVAC) system into a defrost mode of operation, are employed after a fogging condition has developed, and typically require some time to alleviate the fogging condition. As will be appreciated, such systems are not suitable for automatic control, and do not anticipate impending fogging conditions.
In vehicles incorporating an automatic HVAC control system, it has been proposed to sense internal and external conditions to determine whether fogging is likely. Such known systems include a glass temperature sensor and a humidity sensor. The glass temperature sensor is affixed to an internal surface of the windshield glass, and the humidity sensor is disposed adjacent to the windshield glass. The system uses the sensed glass temperature and the sensed cabin humidity to determine whether a possible fogging condition exists, and actuates the HVAC system in response thereto. Unfortunately, such automated systems require additional sensors, which are expensive to purchase and require significant amounts of labor to install and electrically connect to the HVAC controller.
Thus, in order to overcome the above mentioned disadvantage it has been proposed to use existing sensors to calculate the temperature of the glass, which reduces the number of sensors required thereby ultimately reducing the cost of the vehicle. This method is disclosed in U.S. application Ser. Nos. 10/881,347 and 10/881,406, which are both assigned to the assignee of the present invention and are both herein incorporated by reference. The glass temperature in U.S. application Ser. Nos. 10/881,347 and 10/881,406 are calculated from data obtained from existing sensors that measure the speed of the vehicle and the ambient temperature. The method disclosed in U.S. application Ser. Nos. 10/881,347 and 10/881,406 also use a humidity sensor to determine the relative humidity inside the vehicle. These humidity sensors, however, have difficulty measuring or sensing low humidity levels, such as when the relative humidity is less than 10%. Thus, the method disclosed in U.S. application Ser. Nos. 10/881,347 and 10/881,406 limits the intensity of the HVAC response in low humidity conditions.
Therefore, there is a need in the art for a system and method that can accurately determine the humidity when the relative humidity is less than 10% and thereby anticipate fogging conditions under low humidity conditions.